Henry Lin, Department of Electrical and Computer Engineering, California State University, Bakersfield Lecture 3 (Electric Circuits) July 16 th, 2013 1
What is an electrical circuit? An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, voltage sources, current sources, and switches. An electrical circuit is a network consisting of a closed loop, giving a return path for the current. Book Light Circuit 2
What is a resistor? A resistor provides electrical resistance. It limits the flow of electrical current in an electronic circuit. Current is the flow of electricity. 3
What is a capacitor? A capacitor stores energy electrostatically in an electric field. 4
What is a voltage source? A voltage source gives you voltage or electrical potential difference. An example of a voltage source is a battery. 5
How can we arrange the components? The circuit can be in series, parallel, or a combination. In series circuits, current can only take one path. The amount of current is the same at all points in a series circuit. 6
In series 7
Adding resistances in series Each resistance in a series circuit adds to the toal resistance of the circuit. 8
Adding resistances in series 9
Total resistance in a series circuit Light bulbs, resistors, motors, and heaters usually have much greater resistance than wires and batteries. 10
Ohm s Law 11
Calculate current How much current flows in a circuit with a 1.5- volt battery and three 1 ohm resistances (bulbs) in series? 12
Voltage in a series circuit Each separate resistance creates a voltage drop as the current passes through. As current flows along a series circuit, each type of resistor transforms some of the electrical energy into another form of energy. Ohm s law is used to calculate the voltage drop across each resistor. 13
Kirchoff s Voltage Law 14
Parallel circuits In parallel circuits, the current can take more than one path. Because there are multiple branches, the current is not the same at all points in a parallel circuit. 15
In parallel 16
Parallel circuits Sometimes these parallel paths are called branches. The current through a branch is also called the branch current. When analyzing a parallel circuit, remember that the current always has to go somewhere. The total current in the circuit is the sum of the currents in all the branches. At every branch point the current flowing out must equal the current flowing in. This rule is known as Kirchoff s current law. 17
Kirchoff s Current Law 18
Voltage and current in a parallel circuit In a parallel circuit the voltage is the same across each branch. The amount of current in each branch in a parallel circuit is not necessarily the same. The resistance in each branch determines the current in that branch. 19
Advantages of parallel circuits Parallel circuits have two big advantages over series circuits: Each device in the circuit sees the full battery voltage. Each device in the circuit may be turned off independently without stopping the current flowing to other devices in the circuit. 20
Short circuit A short circuit is a parallel path in a circuit with zero or very low resistance. Short circuits can be made accidentally by connecting a wire between two other wires at different voltages. Short circuits are dangerous because they can draw huge amounts of current. 21
Calculate current Two bulbs with different resistances are connected in parallel to batteries with a total voltage of 3 volts. Calculate the total current supplied by the battery. 22
Resistance in parallel circuits Adding resistance in parallel provides another path for current, and more current flows. When more current flows for the same voltage, the total resistance of the circuit decreases. This happens because every new path in a parallel circuit allows more current to flow for the same voltage. 23
Adding resistances in parallel 24
Adding resistance in parallel circuits A circuit contains a 2 ohm resistor and a 4 ohm resistor in parallel. Calculate the total resistance of the circuit. 25
Network circuits In many circuits, resistors are connected both in series and in parallel. Such a circuit is called a network circuit. There is no single formula for adding resistors in a network circuit. For very complex circuits, electrical engineers use computer programs that can rapidly solve equations for the circuit using Kirchoff s laws. 26
Calculate using network circuits Three bulbs each with a resistance of 3Ω, are combined in the circuit in the diagram. Three volts are applied to the circuit. Calculate the current in each of the bulbs. 27
Analysis of circuits 28
Analysis of circuits All circuits work by manipulating currents and voltages. The process of circuit analysis means figuring out what the currents and voltages in a circuit are, and also how they are affected by each other. Three basic lawas are the foundation of circuit analysis. 29
Three circuit laws 30
Voltage divider circuit 31
Voltage divider A circuit divides any supplied voltage by a ratio of the resistors. 32
Solving circuit problems Identify what the problem is asking you to find. Assign variables to the unknown quantities. Make a large clear diagram of the circuit. Label all of the known resistances, currents, and voltages. Use the variables you defined to label the unknowns. You may need to combine resistances to find the total circuit resistance. Use multiple steps to combine series and parallel resistors. 33
Solving circuit problems If you know the total resistance and current, use Ohm s law as V=IR to calculate voltages or voltage drops. If you know the resistance and voltage, use Ohm s law as I=V/R to calculate the current. An unknown resistance can be found using Ohm s law as R=V/I, if you know the current and the voltage drop through the resistor. Use Kirchoff s current and voltage laws as necessary. 34
Solving circuit problems 35
Electric power, AC, and DC electricity How much does electricity cost and what do you pay for? 36
Electric power, AC, and DC electricity The watt (W) is a unit of power. Power is the rate at which energy moves or is used. Since energy is measured in joules, power is measured in joules per second. One joule per second is equal to one watt. 37
Reviewing terms 38
Power in electric circuits One watt is a pretty small amount of power. In everyday use, larger units are more convenient to use. A Kilowatt (kw) is equal to 1,000 watts. The other common unit of power often seen on electric motors is the horsepower. One horsepower is 746 watts. 39
Power 40
Calculate power A light bulb with a resistance of 1.5Ω is connected to a 1.5- volt battery in the circuit shown. Calculate the power used by the light bulb. 41
Paying for electricity Electric companies charge for the number of kilowatt-hours used during a set period of time, often a month. One kilowatt-hour (kwh) means that a kilowatt of power has been used for one hour. Since power multiplied by time is energy, a kilowatthour is a unit of energy. One kilowatt-hour is 3.6 x 10 6 joules. 42
Calculate power Your electric company charges 14 cents per kilowatt-hour. Your coffee maker has a power rating of 1,050 watts. How much does it cost to use the coffee maker one hour per day for a month? 43
Alternating and direct current The current from a the same direction. battery is always in One end of the battery is positive and the other end is negative. The direction of positive to negative. This is called direct current flows from current, or DC. 44
Alternating and direct current If voltage alternates, When the voltage is in the circuit is When the voltage is is the opposite This type of current current, or AC. so does current. positive, the current clockwise. negative the current direction. is called alternating 45
Alternating and direct current AC current is used for almost all high-power applications because it is easier to generate and to transmit over long distances. The 120 volt AC (VAC) electricity used in homes and businesses alternates between peak values of +170V and -170V at a frequency of 60 Hz. AC electricity is usually identified by the average voltage, (120 VAC) not the peak voltage. 46
Voltage vs. Time 47
Power in AC circuits For a circuit containing a motor, the power calculation is a little different from that for a simple resistance like a light bulb. Because motors store energy and act like generators, the current and voltage are not in phase with each other. The current is always a little behind the voltage. 48
Power for AC circuits Electrical engineers use a power factor (pf) to calculate power for AC circuits with motors 49
Application: Wiring in Homes and Buildings 50
Application: Wiring in Homes and Buildings 51
52